1. Field of the Invention
The present invention relates to a slip ring for use in detecting the rotation of an armature of a motor using a pulse signal, and more particularly to such a slip ring having a balanced center of gravity.
2. Description of the Related Art
Motors which require detection of the rotation of an armature are known. In such motors, an armature shaft has on it a rotation-detecting slip ring. The rotation-detecting slip ring usually consists of a conducting portion which extends along the outer periphery of the slip ring over an angle of about 210 degrees and a nonconducting portion which extends over the remaining angle of about 150 degrees. A pulse signal is generated and rotation is thereby detected by alternately providing a conducting state in which two brushes are in contact with the conducting portion and a non-conducting state in which either of the brushes is in contact with the non-conducting portion.
The slip ring is generally composed of a lightweight insulating resin material and a conductor which is made of a heavy-weight material, such as copper, and which covers a portion of the surface of the insulating resin material. In the conventional slip ring, the conductor exists only in the conducting portion of the slip ring, and the center of gravity of the slip ring is thus shifted from the axis of the armature shaft. This provides imbalanced rotation of the slip ring and thus prevents smooth rotation thereof. Hence, to provide balanced rotation, putty is conventionally attached to the rotor core as a balancer. However, balancing by use of putty requires skill and thus greatly complicates the manufacturing process. Moreover, any protrusion of the putty from the diameter of the armature makes the armature useless.